1. Field of the Invention
The present invention relates to the exploration for hydrocarbons and, more particularly, to obtaining core from a wellbore and promptly analyzing the core at the well site to develop a better understanding of the earth's subsurface.
2. Description of the Prior Art
In the exploration for hydrocarbons, the search is concentrated on locating sedimentary rocks which have produced, contained or trapped hydrocarbons. To this end, first of all an understanding of the earth's subsurface is developed for a particular area of interest, usually from commercially available large area survey services. Such surveys can include gravity surveys, magnetic surveys and seismic surveys. Gravity and magnetic surveys are attractive because one can obtain large areal coverage for relatively minimal cost.
Problems with gravity and magnetic surveys are that each suffers from nonuniqueness, lack of resolution and each only provides an estimate of the relative thickness of material that lies above a layer of material with a higher density or magnetic response. Further, these surveys provide little information as to what type of material is below the subsurface or what is below the layer of material with the higher density or magnetic response. A problem with seismic surveys is that the traveltime between source initiation, reflection and receipt is known but the depth to the reflecting points is only an estimate based upon an assumed velocity that the seismic energy travels through each layer of the earth's subsurface. If any one velocity estimate is incorrect, then the depth estimates and the understanding of the subsurface can be greatly altered.
If the first understanding of the earth's subsurface indicates possible sediments, other sources of information are used to refine this understanding, such as geological descriptions of surface rocks and rock outcrops and subsurface information from any adjacent mines and/or previously drilled wellbores. From this refined understanding, a well location is made and drilling of the well is commenced. During and after the well has been drilled, a series of wireline wellbore logs are usually obtained, such as gamma ray, pulsed neutron and resistivity logs, that are used to estimate the mineralogy of the subterranean formations, the presence of hydrocarbons, and inferred rock properties, such as permeability and porosity.
It is recognized that the desired measurement of the physical properties of the earth's subsurface can be best understood by analyzing actual rock samples of the earth's subsurface formations. Such rock samples can be obtained through commercially-available coring services. One of such coring service that has been used extensively for mineral exploration and occasionally for hydrocarbon exploration is described in "Continuous Wireline Core Drilling: An Alternative Method for Oil and Gas Exploration" R. E. Swayne, Drill Bits, Spring 1988 drill bits, pages 7-12. This article also mentions that direct correlations can be made by comparing wireline well logs obtained from the well to measurements made on core samples over the entire length of the well.
Several uses of information obtained from core analysis are described in "Reservoir Description: What Is Needed and When?" by Richardson, et al., published in Symposium on Geology and Reservoir Management, 1986, National Conference on Earth Science, September 1986.